The present invention relates to a dynamic semiconductor memory device. More particularly, the present invention relates to means for measuring the reference level in a sense amplifier of a dynamic semiconductor memory device.
In a dynamic semiconductor memory device, since charges stored by writing information into a real cell leak with the lapse of time and information "1" or information "0" is reversed at the read-out operation, it is necessary to carry out a refresh cycle so as to maintain the information at "1" or "0". Incidentally, when information "1" is maintained, it is not always necessary to charge a capacitor of a memory cell to a level corresponding to the power source voltage. More specifically, in a dynamic memory device comprising a memory cell of a one-transistor one-capacitor type, the transistor acting as a transfer gate is turned on and the sensing operation is carried out by connecting a capacitor of one real cell and a capacitor of one dummy cell to one of a pair of bit lines connected to a sense amplifier. Accordingly, the charge on the real cell may be discharged to such a level that information "1" is detected by the sense amplifier. This minimum charging state (voltage) of the cell capacitor, allowing the sense amplifier to detect information "1", defines the reference level.
Therefore, the reference level depends on the reduction of the potential level of the bit line, that is, the point of the charge state of the capacitor at which information "1" is substantially reversed to information "0". Measurement of the reference level is important for deciding whether or not a refresh cycle is necessary. According to the conventional technique, the reference level is measured by changing the potential of the counter electrode of the capacitor. That is, at the time of testing, the voltage applied to the counter electrode is increased to V.sub.cc (ordinarily 5 Volts)+.DELTA.V, and after the information "1" has been written into the real cell, the level of the counter electrode is returned to V.sub.cc and it is determined whether or not information "1" can be read out in this state. For example, if information "1" can be read out even when the applied voltage is increased to 8 Volts (.DELTA.V=3 V), this means that even if the charge voltage of the capacitor of the real cell is reduced to 3 Volts from V.sub.cc (5 Volts) during normal operation, information "1" can be read out.
However, according to the conventional method, since the power source voltage in the entire circuit, inclusive of the peripheral circuits, is increased to V.sub.cc +.DELTA.V, unfavorable influences on the peripheral circuits and other circuits in which the rated voltage V.sub.cc (i.e., 5 Volts) is used, cannot be avoided.